1. Field of the Invention
The present invention relates generally to a metallic powder for sintering and to sintered metallic powder as well as to a method therefor.
2. Description of the Related Art
Metal injection molding (MIM) is a process for forming metal parts by injecting fine metal powders mixed with a binder into molds similar to those used in conventional plastic injection molding. After molding, the MIM part is debound, in other words, the binder is removed, and the part is sintered, for example at temperatures of 2,200xc2x0 F. or higher, to fuse the fine powdered particles into a solid shape that retains all of the mold""s features.
Specifically, metal powders are mixed with thermoplastic binders or other binders to form a homogeneous mixture, with approximately 60% volume metal powder and 40% volume plastic. This mixture (referred to as xe2x80x9cfeedstockxe2x80x9d) is first heated until it is able to flow, is then injected under relatively low pressure into a mold cavity, and allowed to cool and solidify and finally is ejected as an intricately shaped part. The part is thus molded at relatively low temperatures and pressures in conventional plastic injection molding machines. The molds are similar to those used in plastic injection molding, with slides and multi-cavity configurations possible. The molded xe2x80x9cgreen partsxe2x80x9d are then thermally processed in two steps. First, the binder is removed by evaporation in an operation called debinding. Alternately, the green part is immersed in a bath to dissolve a majority of the binder and then the part is exposed to ultraviolet light to harden the thermosetting component of the binder. Next, the part is sintered (i.e., heated to a temperature near the alloy melting point) in, for example, a dry hydrogen atmosphere, which densifies the part isotropically. The complex shape of the original molded part is retained throughout this process, and close tolerances can be achieved. Only minor, if any, machining is required as a secondary operation.
Metal injection molding may use a variety of alloys and metals, including stainless steels, soft magnetic alloys, controlled expansion alloys and low alloy steels. Specifically, the MIM process allows for a wide selection of metal alloys, including: stainless steel (including 304, 316, 410, 420, and 17-4PH), copper, alloy steels, molybdenum, tool steels, tungsten, and specialty alloys such as ASTM F15 (Kovar), and ASTM F75 (CoCr xe2x80x9cnickel-freexe2x80x9d).
One example of a stainless steel alloy in use is designated 17-4PH. This material has, for example, the following weight percents of material: Cr=16.5%, Ni=4%, Cu=4%, Cb+Ta=0.3%, C=0.03% max, and Fe for the balance. The 17-4PH alloy delivers the corrosion resistance of a type 304 steel, yet is as strong as type 420 martensitic stainless.
Metal injection molding has been used in the following industries: medical, aerospace, ordnance, automotive, dental/orthodontic, electrical, hardware, and consumer products.
The metal powder for the metal injection molding process is formed by spraying. Specifically, the molten metal is forced through a nozzle to form small droplets. The metal alone results in droplets of a size which is too large to serve as an injection molding powder and so silicon is added to about 1% to the melted metal to aid the flow of the liquid metal through the nozzle and form smaller droplets. These small droplets cool to form the metal powder used for the metal injection molding.
The present invention in one aspect provides a metal injection molding feedstock having improved properties, in particular, a metal injection molding feedstock powder having a reduced amount of silicon. The invention in another aspect also provides a metal injection molded part having little or no agglomerated silica. The invention in yet another aspect also relates to a method for forming powdered metal for metal injection molding by reducing the quantity of silicon.